The present invention relates in general to communicating over power lines and more particularly concerns novel techniques and apparatus for exchanging digital data over distribution power lines between a central control unit and remote meter or control modules. The invention is especially useful in facilitating automatically monitoring and controlling the power distribution system and reading meters at remote locations with good reliability. It accomplishes this using the existing power distribution system between a substation where a typical central control unit is located and each remote meter or control module not only as a transmission path, but also as a synchronization source. The invention advantageously incorporates fewer components at the remote meter and control modules than at the central control unit to help keep system costs down.
Utility meters are typically read periodically by a meter reader at the location of each customer who manually records each meter reading and returns the information concerning the date, location and reading to a central office. At the central office a keypuncher or other operator manually converts this data into a form for automatic processing by billing computers.
When meters are inside a building and no one is available to admit the meter reader, the meter reader does not record the reading for that period. The meter reader may leave a post card addressed to the utility instructing the customer to read the meter himself, write the reading on the postcard and mail the reading to the utility. Alternatively, the utility may estimate the use for that period based on past history. Both of these approaches are subject to inaccuracy. Moreover, even in systems where the meter is located outside and the meter reader always has access to it, manually obtaining and converting these readings for automatic processing is costly, time consuming and subject to error in each step between initial reading by the meter reader and automatic data processing for billing by the computer.
A number of automatic meter reading systems have been proposed. One approach involves the use of telephone lines to carry the data. Another comtemplates transceivers at each customer location with an aircraft flying over the area to interrogate transponders at each customer location through the transceivers. Still another approach contemplates the use of power lines for communicating data, but with costly links bypassing each distribution transformer. A disadvantage common to all these systems is high cost.
Utilities presently monitor the power flow on distribution lines at very few locations because of the high cost of telephone lines or limited radio-frequency channel allocation. Control elements such as switches are manually operated while power factor correction capacitors are operated by a clock rather than in response to load demands. When power outages occur because of breaks in the distribution line, utilities wait for customer telephone complaints to inform them of the affected area because they have no method of monitoring or controlling the distribution system in real time.
Utilities also have to provide generating capacity to meet the peak load demands, rather than the average or essential load. The present method of peak load shaving is to lower the distribution voltage, which is harmful to items such as computers and air conditioners, or in the extreme, to black out sections of their franchised area by interrupting service. It is a far better practice to shave peak load by interrupting service to non-essential power loads such as hot water heaters. This is presently accomplished by using time clocks, which lose time during power outages and therefore eventually do not remove the load during periods of maximum power consumption, or by general appeals to the public to turn off non-essential loads.